The microphone and the Camera have in common that recording, storage and
playback are already useful (in fact the same holds for the pen, too).
Because audio has lower demands than video, much more is possible. Some
application could be controlled by speech recognition. Or the incoming
sound itself could be processed in some way (filtered).

The most useful application of sound in computers will be speech
recognition. Several methods already proved to be capable of doing
reasonable recognition in real-time. Especially the Hidden Markov Model
(HMM) is becoming popular.